As multimedia applications increase the demand for high-bandwidth, high-bit-rate communications, fiber optics technology is rapidly advancing to supply the capacity. A family of standards for optical fiber transmissions is known as the Synchronous Optical Network (SONET) standards. SONET was born as an extension to the DS1 hierarchy, which is a hierarchy of "electrical" as opposed to "optical" signals and consists of levels of signals formed by multiplexing lower level TDM (time division multiplex) signals.
The SONET standard establishes a multiplexing format for using any number of 51.84 Mbits/s signals as building blocks. An OC-3 (Optical Carrier, Level 3) is a 155.52 Mbits/s signal (3.times.51.84 Mbits/s), and its electrical signal counterpart is referred to as an STS-3 signal. The STS-1 signal carries a DS3 signal or a number of DS1 or other lower level signals. A SONET STS-3 signal is created by concatenating STS-1 signals. Each SONET STS-N electrical signal has a corresponding OC-N "optical signal". The OC-N signals are created by converting the STS-N electrical signal to an optical signal.
Although optical switching techniques have been developed, telecom companies are eager to provide as much performance as possible from their existing infrastructure. Switching systems based on the DS1 electrical signal hierarchy are in place and continue to be used for signals carrying that type of signal. Essentially these switching systems use DS0 data, which is derived from the DS1 hierarchy. For example, a DS1 signal is comprised of 24 multiplexed DS0 voice channels. Thus, there is a demand for interfaces that will permit SONET signals to be switched through switching systems designed for the DS1 hierarchy of signals.